The present invention relates to a starter-generator system and, more particularly, a high frequency starter-generator system without a rotating diode rectifier installed on the rotor.
Many aircraft include AC generator systems to supply relatively constant frequency or variable frequency AC power. Many of the AC generator systems installed in aircraft include three separate brushless generators, namely, a permanent magnet generator (PMG), an exciter, and a main generator. The PMG includes a rotor having permanent magnets mounted thereon, and a stator having a plurality of windings. When the PMG rotor rotates, the permanent magnets induce AC currents in PMG stator windings. These AC currents are typically fed to a regulator or a control device, which in turn outputs a DC current to the exciter.
The exciter typically includes single-phase (e.g., DC) stator windings and multi-phase (e.g., three-phase) rotor windings. The DC current from the regulator or control device is supplied to exciter stator windings, and as the exciter rotor rotates, three phases of AC current are typically induced in the rotor windings. Rectifier circuits that rotate with the exciter rotor rectify this three-phase AC current, and the resulting DC currents are provided to the main generator field winding installed on the rotor. The main generator additionally includes a rotor and a stator having single-phase (e.g., DC) and multi-phase (e.g., three-phase) windings, respectively. The DC currents from the rectifier circuits are supplied to the rotor windings. Thus, as the main generator rotor rotates, three phases of AC current are induced in main generator stator windings. This three-phase AC current can then be provided to a load such as, for example, electrical aircraft system.
The aircraft industry has been undergoing significant change as a result of advances in the capability of power electronics. Secondary power functions to start main engines and provide power to environmental control systems were previously supplied by high pressure (bleed air) pneumatic systems, and are now being supplied by electric power systems. To supply the additional electric power required for the new functions, the generating and distribution systems have abandoned the traditional fixed frequency power standard in favor of variable frequency or high voltage dc electric systems.
Existing more electric aircraft are based on variable frequency AC systems generally described in industry standards over the range of 360 to 800 Hz. This standard accommodates the typical variation in main engine driven generator frequency and allows the same aircraft bus to operate with a fixed frequency generator, such as typically supplied from the auxiliary power unit (APU). However, this system imposes design constraints on the generating and conversion equipment, since the electromagnetic design is heavily dependent upon the minimum frequency, which leads to weight and volume penalties on the power generation and conversion equipment.
Using higher frequency generators that produce from 800 to 1,600 Hz have been proposed to address this issue. These designs typically use a larger number of poles on the rotors on typical brushless configurations. The AC power is typically rectified and DC power is distributed to the loads.
As can be seen, there is a need for a starter-generator system that can produce a high frequency output while providing weight and cost saving and increased reliability, as compared to prior art designs.